Feedback amplifier utilizing a feed forward technique to achieve high direct currentgain and wide bandwidth



Oct. 29, 1968 R. ORDOWER 3,408,586 FEEDBACK AMPLIFIER UTILIZING A FEED FORWARD TECHNIQUE TO ACHIEVE HIGH DIRECT C RENT IN AND WIDE BANDWIDTH File uly 1966 INVENTOR ROBERT ORDOWER BY @M A TTORNE Y United States Patent Office 3,408,586 I FEEDBACK AMPLIFIER UTILIZING A: FEED FOR- WARD TECHNIQUE CURRENT GAIN BANDWIDTH Robert Ordower, Vestal, N.Y., assignor to International Busiliress Machines Corporafion, a corporation of New Yor Filed July 19, 1966, SerINo. 566,328 Claims. (Cl. 330 2s This invention relates to an improved direct current amplifier which exhibits simplicity of design,

stability problems. Many feedback amplifier applications require a very high direct current gain in order to obtain niques in order to improve amplifier characteristics has pp. 97-100 of High Speed Analog by T. Karplus, published in 1962 by J.

wide bandwidth response characteristic.

It is a more specific object of the present invention to provide an improved amplifierof the type described in the preceding paragraph which utilizes an improved feed forward technique to achieve the improved gain-bandwidth response characteristics.

(1) The loop gain, of the amplifier did not roll off at a constant, predetermined slope (e.g. twenty decibels/ a poor step response, even F (frequency at which loop gain of the amplifier is db) of the amplifier was very high;

(2) The amplifier was conditionally stable-Le. if there is more than ninety degrees phase shift in the low cur.

These problems are obviated and the above objectsgare Patented Oct. 29, 1968 2 frequency pa th,-a conditional stability problem may ocin a preferred embodiment of the invention pole zero cancellations.

It is therefore an important object of the present invention to provide a very economical and effective high gain, theoretically, which can be open loop gain as desired, without affecting the open loop F The foregoing and other objects, features and advantages of the invention will be apparent from the folpanying drawing.

In the drawing:

FIG. 1 illustrates amplifier; and

FIG. 2 illustrates amplifier.

The improved amplifier illustrated in FIG. 1 includes a plurality of amplifier stages 1-1 to 1-11, each of which is preferably identical to the others. The output of the final stage 1-1: is connected through a buffer stage cona preferred form of the improved an alternative form of the improved positive supply terminal 11, and connected to a negative supply terminal 12 by way of a resistor 13. Capacitors 15-1, 15-2 and 15-n are connected respectively to the input terminals 16-1, 16-2 and 16-22 of the amplifier stages 1-2, 1-22 and 2.

'connects the junction between ground. Negative :20 is'provided by way ,of

' input terminal The e quivalent impedance which sees looking into the electrode is connectedto a positive supply terminal 23 by way of resistors 24 and 25.

the transistor 21 having'a high input impedance and a 30 defines this equivalent impedance at the terminal 16-1 of FIG. 1, since it is in series with the relatively low output impedance of FIG. 2 illustrates an alternative form which each stage such as 1-1 can take. In this form an inductor 50 and a resistor 51 replace the resistor 30 and the capacitor 15-1 of FIG. 1, respectively. The value of the impedance of the inductor increases with increasing input signal frequency, whereby at some frequency it is so much greater than the impedance of the resistor 51 that the high frequency component of the step signal is bypassed around stage 1-1 by means of the emitter follower 10 and the resistor 51.

Although the stages 1-1 to 1-n and the buffer ampliunction 16-1 should be well de- Ifined. This is assured in the preferred embodiment by choosing stages 1 low output impedance. The resistor fier 10 are shown as stage 1-1 and in parallel with the relatively high input impedance of stage 1-2. I v

, In the preferred embodiment, each of the stages 1-2 to 1-n is similar to the stage 1-1 and will not, therefore, be described.

The emitter follower stage 40 has its emitter electrode connected to a negative supply terminal 43 by way of a resistor 41, and its collector electrode is connected to a positive supply terminal 44.

The transistor amplifier 2 has its emitter electrode connected to ground potential 'by way of a resistor 31, and its collector electrode is connected to a positive supply terminal 32 by way of resistors 33 and 34. A decoupling capacitor 35 connects the junction between the resistors 33 and 34 to ground potential.

Utilizing the component values set forth below, the circuit configuration of FIG. 1 obviated the problems encountered by previous unsuccessful attempts to achieve a predetermined constant slope for roll-off and assured stability with a minimum number of components. The open loop gain of the circuit was approximately six hundred-forty thousand and the loop gain F was approxiimately twenty megacycles:

Resistors: Values 3 ohms 500 5 do 1000 13 do 1200 22 do.. 25 24, 33 do 22 25 do 6000 26, 30 do 4000 27 do 1000 31 do 20 34 do 200 41 do 1000 42 do 1000 Capacitors: Values 6 pf 15-1 mfd .37 15-2 mfd .0091 15-11 pf 224 28, 35 mfd .1

noninverting amplifiers, it will be appreciated that inverting amplifiers can be used so long as the inputs to node 161 to 16-n remain in phase. Also a field effect transistor amplifier can be utilized as the buffer 10.

,:Each stage of the direct current amplifiercan be of the other known'constructions, for example, of the differential amplifier type. 4 I

While the inventionhas beenv particularly shown ,and described with reference to a preferred embodime'nt 'thereof, it willbeunderstood bythoseskilled in the art that the foregoing and other" changes in form and details may be made therein without departing from the spirit and scope of the invention."

What is claimed is:

1. A transistor amplifier having a high direct current gain and wide bandwidth characteristic, comprising an input terminal and an output terminal;

a plurality of cascadeconnected transistor voltage amplifierstages connected to the input terminal, each having a high input impedance and a low output impedance;

an output transistor amplifier connecting the last one of said stages to the output terminal,

means providing negative feedback between the output amplifier and the first one of said stages,

a high input impedance buffer having its input connected to the input signal terminal and having an output; and

means, including a first plurality of impedance elements each coupling the output of the buffer to the input of a respective one of said stages other than the first and to the output amplifier and including a second plurality of impedance elements forming output coupling for the stages, and effective to bypass progressively higher frequency components of input signals around a progressively larger number of stages.

2. The combination set forth in claim 1 wherein the first plurality of impedance elements comprises capacitors of selected values; and

the second plurality of impedance elements comprises resistors of selected value.

3. The combination set forth in claim 2 wherein each of said stages is a non-inverting amplifier including a pair of cascaded common emitter amplifiers with second-collector-to-first-emitter feedback; and

wherein the buffer is an emitter follower.

4. The combination set forth in claim 1 wherein the first plurality of impedance elements comprises resistors of selected value; and

the second plurality of impedance elements comprises inductors of selected values.

5. In a circuit of the type in which at least one voltage amplifier stage adapted to receive input signals and an output amplifier are connected in the form of a direct current feedback amplifier, the amplifier stage having an output coupled to an input of the output amplifier,

in combination therewith a feed forward means for increasing the gain-bandwidth product of the feedback amplifier comprising means including a high input impedance buffer having a voltage gain less than unity and a pair of impedance elements causing high frequency components of the input signals to be bypassed around the amplifier stage to the input of the output amplifier.

6. The feedback amplifier of claim 5 wherein the amplifier stage is of the non-inverting type having a high input impedance and a low output impedance,

wherein the buffer comprises an emitter follower; and

wherein one of the impedance elements is a capacitor coupling the buffer output to the output amplifier and the other impedance element is a resistor coupling the output of the amplifier stage to the output amplifier.

7. The feedback amplifier of claim 5 wherein the amplifier stage is of the non-inverting type I having a high input impedance and a low output impedance,

wherein the buffer comprises an emitter follower; and

wherein one of the impedance elements is a resistor coupling the buffer output to the output amplifier and the other impedance element is an inductor coupling the output of the amplifier stage to the output amplifier.

8. The feedback amplifier of claim 6 further comprising a capacitor for each stage other than the first stage and a resistor for each stage output for coupling high frequency components of input signals around preceding stages.

References Cited UNITED STATES PATENTS 2,7S1,423 2/1957 Kuczun et a1 330-126 3,281,705 10/ 1966 Frye 330-30 3,296,464 1/ 1967 Brault 330--126 X ROY LAKE, Primary Examiner.

a plurality of said voltage amplifier stages connected 5 J. B. MULLINS, Assistant Examiner.

in cascade form; and 

